Strong, uniform magnetic fields are desirable in many situations. Examples include scientific instruments, devices for measuring important industrial parameters, apparatus for achieving certain effects on samples or materials, devices for education or student experimentation in instructional settings, and many others. Often, a device or phenomenon that relies on or incorporates a magnetic field in its operation will be improved if the field is more uniform in strength. Such improvements may be an increase in speed, efficiency, precision, resolution, stability, sensitivity, or other feature or the device, measurement, or process. A more uniform magnetic field may also allow the use of smaller, weaker, and/or less expensive magnets to achieve a desired goal.
In other cases, improvements in device performance may result from a magnetic field having a particular non-uniform field strength distribution throughout a region of space. Often, and in particular for the present disclosure, methods for homogenizing magnetic fields can also be used to accurately achieve particular non-uniform target field distributions.
Many methods for homogenizing or correcting the magnetic field of magnets have been developed. Mechanical methods, including precise design and shaping of the magnet's pieces, attention to accurate positioning during the assembly of the magnet, careful selection of uniform and/or matched pieces of permanent magnetic material, and careful adjustment of the relative positions of the magnet pieces after initial assembly, have been used. Post-assembly mechanical adjustments, including screws to move some or all of the positions of the magnet's constituent pieces, movement of magnet pieces or other structures that control the distribution of magnetic flux lines in space, and large-scale distortions of the magnet structure are all examples of mechanical methods for improving the field homogeneity of magnets. This list of mechanical methods is not exhaustive. For example, magnets may be disassembled and then reassembled with new thin pieces or “shims” incorporated so that the relative positions of the magnet's constituents are altered, changing the shape of the magnetic field. Other examples of past efforts to adjust, with mechanical shimming, a magnetic field include U.S. Pat. No. 4,631,481 to Young et al., U.S. Pat. No. 5,235,284 to Tahara et al., U.S. Pat. No. 5,343,183 to Shimada et al., and U.S. Pat. No. 6,313,634 to Kasten.
Of present relevance are techniques generally known as “passive shimming,” in which additions of extra pieces of material on the surface of or in the interior and/or exterior of the magnet are used to change the magnetic field in a manner that does not require electrical current. Passive shimming is frequently achieved through the careful placement of small pieces (known as “buttons”) of highly magnetizeable materials (low-carbon steel, nickel, or similarly magnetically responsive materials) or of permanent magnet materials such as neodymium-iron-boron, samarium-cobalt, or other like materials known in the art. The buttons are discrete pieces of various sizes, and the locations, orientation, sizes, and number of buttons needed to correct the field are determined through the use of magnetic field measurements, magnetic field calculations or simulations, or combinations of these and other techniques well known in the art (see, as an example, U.S. Pat. No. 5,045,794 to Dorri). The buttons are typically much smaller than the size scale of the magnet, its gap, and the working volume over which field correction is desired.
Non-mechanical methods are likewise possible. The non-mechanical methods often have the attractive characteristic of continuous adjustability, ease of adjustment, and ease of automation. One common non-mechanical method utilizes wires that carry electrical current. Such currents create magnetic fields in the region of space around the wires. These fields are strongest near the wires and have a spatial distribution of strength and orientation. By carefully selecting the locations and adjusting the currents in such wires, the magnetic field can be made more homogeneous because the magnetic fields due to the currents can counteract some or all of the nonhomogeneities present in the field of the original magnet. By analogy with the thin sheets or strips of metal (e.g., shim stock) that have sometimes been used to mechanically adjust magnets, the coils of wire have come to be known as “shims” or “shim coils.” Various methods known in the art may be used to optimize the positions of the wires. Further methods known in the art may be used to optimize the current values in each shim coil in order to achieve the desired field correction, a process referred to as “shimming the magnet.”
Non-mechanical methods may be used in conjunction with one or more of the mechanical methods. Mechanical adjustments are often preferred for making large corrections, since these can be made without requiring large electrical currents which may be detrimental to the magnet performance. On the other hand, non-mechanical adjustments are often preferred for fine correction of the residual field defects because currents can be adjusted in a continuous manner to achieve the required fine adjustment.